all Pontiac blocks and heads are the same size, 301 all the way up to a 455. bore and stroke and such are whats different. back in the day gm brands had their own engines. caddy had their own, buick, chevy and olds, all very different engines. nothing other then the carbs ( good old quardra bogs) could be swapped from one to the other.

so technically, no, you cant get a big block. cause it dont exist. they did make 455 HO but its just a performance stamp.